U.S. patent number 9,642,237 [Application Number 14/282,805] was granted by the patent office on 2017-05-02 for method of improving electrode life by simultaneously controlling plasma gas composition and gas flow.
This patent grant is currently assigned to Hypertherm, Inc.. The grantee listed for this patent is Hypertherm, Inc.. Invention is credited to Jon W. Lindsay, John Peters.
United States Patent |
9,642,237 |
Peters , et al. |
May 2, 2017 |
Method of improving electrode life by simultaneously controlling
plasma gas composition and gas flow
Abstract
A method of operating a plasma arc torch system is provided. A
first plasma gas supply source, a second plasma gas supply source,
and a control unit are provided. A first plasma gas composition is
flowed through a first plasma gas flow path, and a plasma arc is
generated using the first plasma gas composition. After arc
generation, the plasma gas composition is changed to a second
plasma gas composition, and the plasma gas flow path is changed to
a second plasma gas flow path, wherein the second plasma gas flow
path is different from the first plasma gas flow path. The plasma
arc is sustained using the second plasma gas composition. The first
and second plasma gas flow paths are both at least partially
disposed within the plasma arc torch.
Inventors: |
Peters; John (Canaan, NH),
Lindsay; Jon W. (Hanover, NH) |
Applicant: |
Name |
City |
State |
Country |
Type |
Hypertherm, Inc. |
Hanover |
NH |
US |
|
|
Assignee: |
Hypertherm, Inc. (Hanover,
NH)
|
Family
ID: |
54150641 |
Appl.
No.: |
14/282,805 |
Filed: |
May 20, 2014 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20150342019 A1 |
Nov 26, 2015 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H05H
1/3405 (20130101); B23K 10/006 (20130101); H05H
1/34 (20130101); H05H 1/3421 (20210501); H05H
1/3468 (20210501); H05H 1/3494 (20210501) |
Current International
Class: |
B23K
10/00 (20060101); H05H 1/34 (20060101) |
Field of
Search: |
;219/121.5,121.52,121.51,121.48,75,121.54,121.55,121.59 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Paschall; Mark
Attorney, Agent or Firm: Proskauer Rose LLP
Claims
What is claimed is:
1. A method of operating a plasma arc torch system, the method
comprising: providing (i) a plasma arc torch having a plasma
chamber in which a plasma arc is generated, (ii) a first plasma gas
supply source and a second plasma gas supply source for providing a
plasma gas flow to the plasma arc torch, and (iii) a control unit
for controlling cutting parameters including plasma gas composition
and plasma gas flow path; flowing a first plasma gas composition
through a first plasma gas flow path during an arc ignition period;
generating the plasma arc using the first plasma gas composition;
changing the first plasma gas composition to a second plasma gas
composition after the plasma arc is generated; changing the first
plasma gas flow path to a second plasma gas flow path after the
plasma arc is generated, the second plasma gas flow path different
from the first plasma gas flow path; and sustaining the plasma arc
using the second plasma gas composition during a cutting period;
wherein the first and second plasma gas flow paths both are at
least partially disposed within the plasma arc torch.
2. The method of claim 1 further comprising cutting a workpiece
with the plasma arc torch after the changing of the first plasma
gas composition to the second plasma gas composition and the first
plasma gas flow path to the second plasma gas flow path.
3. The method of claim 1 further comprising adjusting, in response
to receiving a stop signal, at least one of a supply current, a gas
flow pressure, a gas composition, or a gas flow path.
4. The method of claim 1 further comprising detecting an arc
transfer after the plasma arc is generated.
5. The method of claim 1 further comprising at least one of (i)
changing the second plasma gas composition to a third plasma gas
composition; or (ii) changing the second plasma gas flow path to a
third plasma gas flow path.
6. The method of claim 5 wherein the third plasma gas flow path
includes a portion of the first plasma gas flow path and a portion
of the second plasma gas flow path.
7. The method of claim 5 wherein changing the second plasma gas
composition or the second plasma gas flow path is performed in
response to either (i) a removal of a start signal from the plasma
torch system; or (ii) a time offset from a transfer sense.
8. The method of claim 5 wherein changing the second plasma gas
composition or the second plasma gas flow path includes decreasing
a gas pressure of the plasma gas.
9. The method of claim 5 further comprising adjusting a current
value of the plasma arc.
10. The method of claim 1 wherein the first plasma gas flow path is
configured to impart a substantially radial velocity component to
the plasma gas.
11. The method of claim 1 wherein the second plasma gas flow path
is configured to impart a substantially axial velocity component to
the plasma gas.
12. The method of claim 1 wherein the first plasma gas composition
includes at least one of oxygen, nitrogen, air, or argon.
13. The method of claim 1 wherein the second plasma gas composition
includes an oxidizing gas or consists essentially of oxygen.
14. The method of claim 1 wherein changing the first plasma gas
flow path to a second plasma gas flow path includes changing at
least one of a plasma gas pressure or a plasma gas flow rate.
15. The method of claim 1 wherein the first plasma gas flow path
and the second plasma gas flow path are configured to impart
different and distinct velocity components or swirl patterns to the
plasma gas flow through the plasma arc torch.
16. The method of claim 5 wherein the third plasma gas flow path is
configured to impart a third swirl pattern to the plasma gas
flow.
17. A method for operating a plasma torch system, the method
comprising: providing (i) a plasma torch having a nozzle and an
electrode, the nozzle and the electrode defining a plasma chamber
in which a plasma arc is generated, (ii) a plasma gas supply source
for providing a plasma gas flow to the plasma torch, and (iii) a
control unit for controlling cutting parameters including a plasma
gas composition and a plasma gas flow path to the plasma torch;
generating the plasma arc; establishing a first plasma gas
composition and a first plasma gas flow path during an arc ignition
period, the first plasma gas flow path configured to impart a first
swirl pattern to the plasma gas flow; changing the first plasma gas
composition to a second plasma gas composition; changing the first
plasma gas flow path to a second plasma gas flow path, the second
plasma gas flow path configured to impart a second swirl pattern to
the plasma gas flow during a cutting period; changing the second
plasma gas composition to a third plasma gas composition; and
changing the second plasma gas flow path to a third plasma gas flow
path, the third plasma gas flow path configured to impart a third
swirl pattern on the plasma gas flow.
18. The method of claim 17 wherein the first gas composition and
the third gas composition are substantially similar.
19. The method of claim 17 wherein the third plasma gas flow path
and the first plasma gas flow path are substantially similar.
20. The method of claim 17 wherein the first swirl pattern is
distinct from at least one of the second swirl pattern or the third
swirl pattern.
21. The method of claim 17 wherein changing the first plasma gas
flow path includes changing a flow rate of plasma gas to the plasma
arc torch.
22. The method of claim 17 wherein changing the second plasma gas
flow path includes changing a flow rate of plasma gas to the plasma
arc torch.
Description
FIELD OF THE INVENTION
The invention relates generally to the field of plasma arc cutting
systems and processes. More specifically, the invention relates to
methods and apparatuses for improving electrode life of a plasma
cutter by simultaneously controlling the gas composition and the
gas flow pattern around the electrode.
BACKGROUND
Plasma arc torches are widely used in the cutting and marking of
materials. A plasma torch generally includes an electrode and a
nozzle having a central exit orifice mounted within a torch body,
electrical connections, passages for cooling, and passages for arc
control fluids (e.g., plasma gas). The torch produces a plasma arc,
a constricted ionized jet of a gas with high temperature and high
momentum. Gases used in the torch can be non-reactive (e.g., argon
or nitrogen) or reactive (e.g., oxygen or air). During operation, a
pilot arc is first generated between the electrode (cathode) and
the nozzle (anode). Generation of the pilot arc can be by means of
a high frequency, high voltage signal coupled to a DC power supply
and the torch or by means of any of a variety of contact starting
methods.
In plasma cutting systems, the gas flow needed for plasma cutting
can be different from the gas flow that is needed for arc ignition.
At ignition the gas swirl strength may need to be increased or
decreased to provide a stable arc with low erosion, but during
cutting the optimum flow rate and swirl strength are often
different. Known plasma cutting systems do not permit both the gas
chemistry and the gas flow pattern to be optimized independently of
each other between and/or during arc ignition, cutting and arc
extinction. Optimizing one parameter often requires compromising
with respect to another and can cause electrode life to decrease as
a result.
SUMMARY OF THE INVENTION
The present invention addresses the unmet need for a plasma arc
cutting system that enables both the gas flow chemistry and gas
flow pattern to be optimized independently of one another during
and between the stages of arc ignition, cutting and arc extinction.
Specifically, the present invention relates to systems and methods
for establishing in a plasma arc cutting system optimal gas flow
chemistries (e.g. gas composition) and gas flow patterns (e.g.
swirl patterns around the electrode) independently of one another,
e.g. able to vary depending on the stage of a cutting operation
(e.g. arc ignition, cutting, arc extinction). The present invention
optimizes consumable life and achieves maximum cut performance
without sacrificing optimum flow pattern for the sake of optimum
gas chemistry, or vice versa.
In one aspect, the invention features a method of operating a
plasma arc torch system. The method includes providing a plasma arc
torch having a plasma chamber in which a plasma arc is generated.
The method includes providing a first plasma gas supply source and
a second plasma gas supply source for providing a plasma gas flow
to the plasma arc torch. The method includes providing a control
unit for controlling cutting parameters including plasma gas
composition and plasma gas flow path. The method includes flowing a
first plasma gas composition through a first plasma gas flow path.
The method includes generating the plasma arc using the first
plasma gas composition. The method includes changing the first
plasma gas composition to a second plasma gas composition after the
plasma arc is generated. The method includes changing the first
plasma gas flow path to a second plasma gas flow path after the
plasma arc is generated, the second plasma gas flow path different
from the first plasma gas flow path. The method includes sustaining
the plasma arc using the second plasma gas composition. The first
and second plasma gas flow paths both are at least partially
disposed within the plasma arc torch.
In some embodiments, the method includes cutting a workpiece with
the plasma arc torch after changing the first plasma gas
composition to the second plasma gas composition and after changing
the first plasma gas flow path to the second plasma gas flow path.
In some embodiments, the method includes adjusting, in response to
receiving a stop signal, at least one of a supply current, a gas
flow pressure, a gas composition, or a gas flow path. In some
embodiments, the method includes detecting an arc transfer after
the plasma arc is generated. In some embodiments, the method
includes at least one of (i) changing the second plasma gas
composition to a third plasma gas composition; and/or (ii) changing
the second plasma gas flow path to a third plasma gas flow
path.
In some embodiments, the third plasma gas flow path includes a
portion of the first plasma gas flow path and/or a portion of the
second plasma gas flow path. In some embodiments, changing the
second plasma gas composition and/or the second plasma gas flow
path is performed in response to (i) a removal of a start signal
from the plasma torch system; and/or (ii) a time offset from a
transfer sense. In some embodiments, changing the second plasma gas
composition or the second plasma gas flow path includes decreasing
a gas pressure of the plasma gas. In some embodiments, the method
includes adjusting a current value of the plasma arc.
In some embodiments, the first plasma gas flow path is configured
to impart a substantially radial velocity component to the plasma
gas. In some embodiments, the second plasma gas flow path is
configured to impart a substantially axial velocity component to
the plasma gas. In some embodiments, the first plasma gas
composition includes at least one of oxygen, nitrogen, air, or
argon. In some embodiments, the second plasma gas composition
includes an oxidizing gas or consists essentially of oxygen. In
some embodiments, changing the first plasma gas flow path to a
second plasma gas flow path includes changing at least one of a
plasma gas pressure or a plasma gas flow rate. In some embodiments,
the first plasma gas flow path and the second plasma gas flow path
are configured to impart different and distinct velocity components
or swirl patterns to the plasma gas flow through the plasma arc
torch. In some embodiments, the third plasma gas flow path is
configured to impart a third swirl pattern to the plasma gas
flow.
In another aspect, the invention features a method for operating a
plasma torch system. The method includes providing a plasma torch
having a nozzle and an electrode. The nozzle and the electrode
define a plasma chamber in which a plasma arc is generated. The
method includes providing a plasma gas supply source for providing
a plasma gas flow to the plasma torch. The method includes
providing a control unit for controlling cutting parameters
including a plasma gas composition and a plasma gas flow path to
the plasma torch. The method includes generating the plasma arc.
The method includes establishing a first plasma gas composition and
a first plasma gas flow path. The first plasma gas flow path is
configured to impart a first swirl pattern to the plasma gas flow.
The method includes changing the first plasma gas composition to a
second plasma gas composition. The method includes changing the
first plasma gas flow path to a second plasma gas flow path. The
second plasma gas flow path is configured to impart a second swirl
pattern to the plasma gas flow. The method includes changing the
second plasma gas composition to a third plasma gas composition.
The method includes changing the second plasma gas flow path to a
third plasma gas flow path. The third plasma gas flow path is
configured to impart a third swirl pattern on the plasma gas
flow.
In some embodiments, the first gas composition and the third gas
composition are substantially similar. In some embodiments, the
third plasma gas flow path and the first plasma gas flow path are
substantially similar. In some embodiments, the first swirl pattern
is distinct from at least one of the second swirl pattern or the
third swirl pattern. In some embodiments, changing the first plasma
gas flow path includes changing a flow rate of plasma gas to the
plasma arc torch. In some embodiments, changing the second plasma
gas flow path includes changing a flow rate of plasma gas to the
plasma arc torch.
In another aspect, the invention features a component for a plasma
arc torch system. The component includes a non-transitory computer
readable product tangibly embodied in an information carrier for
use in a plasma torch system. The computer readable product is
configured to cause a computer to execute a process for cutting a
workpiece. The process includes generating a plasma arc using a
first plasma gas composition and a first plasma gas flow path. The
process includes changing the first plasma gas composition to a
second plasma gas composition after the plasma arc is generated.
The process includes changing the first plasma gas flow path to a
second plasma gas flow path after the plasma arc is generated.
In some embodiments, the process further includes cutting a
workpiece with the plasma arc torch after changing the first plasma
gas composition to the second plasma gas composition and the first
plasma gas flow path to the second plasma gas flow path. In some
embodiments, the process further includes adjusting, in response to
receiving a stop signal, at least one of a supply current, a gas
flow pressure, a gas composition, or a gas flow path. In some
embodiments, the process includes detecting an arc transfer after
the plasma arc is generated. In some embodiments, the process
further includes at least one of (i) changing the second plasma gas
composition to a third plasma gas composition; or (ii) changing the
second plasma gas flow path to a third plasma gas flow path. In
some embodiments, the third plasma gas flow path includes a portion
of the first plasma gas flow path and/or a portion of the second
plasma gas flow path.
In another aspect, the invention features a plasma arc torch
system. The plasma arc torch system includes a plasma arc torch
body including a nozzle and an electrode. The nozzle and the
electrode define a plasma chamber for generating a plasma arc. A
first plasma gas flow path is at least partially disposed within
the plasma arc torch body. The first plasma gas flow path is
configured to independently support a first flow pattern of plasma
gas to the plasma chamber. The first flow pattern is at least
substantially directed radially inward. A second plasma gas flow
path is at least partially disposed within the plasma arc torch
body. The second plasma gas flow path is configured to
independently support a second flow pattern of plasma gas to the
plasma chamber. The second flow pattern is at least substantially
directed axially into a plasma chamber. The second plasma gas flow
path is different from and/or independent of the first plasma gas
flow path.
A first plasma gas supply source is connectable to the first plasma
gas flow path. A second plasma gas supply source is connectable to
the second plasma gas flow path. A control unit is configured to
change the first plasma gas supply source to a second plasma gas
supply source having a second plasma gas composition. The control
unit is configured to direct the second plasma gas composition
through the second plasma gas flow path imparting the second plasma
gas flow pattern on the second plasma gas during a cutting
operation. The control unit is configured to change to the second
plasma gas supply source and the second plasma gas flow path after
an ignition of the plasma arc and/or before an extinction of the
plasma arc.
In some embodiments, a set of valves is disposed between the plasma
torch tip configuration and the first and second plasma gas supply
sources. In some embodiments, the set of valves includes a first
valve configured to direct plasma gas flow to the first plasma gas
flow path, a second valve configured to direct plasma gas flow to
the second plasma gas flow path, and a third valve configured to
direct plasma gas flow to the first plasma gas flow path and/or the
second plasma gas flow path. In some embodiments, the set of valves
includes a first valve configured to direct plasma gas flow to the
first plasma gas flow path and a third valve, a second valve
configured to direct plasma gas flow to the second plasma gas flow
path and the third valve. In some embodiments, the third valve is
configured to direct plasma gas flow to both the first plasma gas
flow path and the second plasma gas flow path simultaneously.
In some embodiments, the plasma arc torch system includes a
non-transitory computer readable product tangibly embodied in the
control unit. In some embodiments, the computer readable product
includes cutting information including instructions to change a
first plasma gas composition to a second plasma gas composition and
to flow plasma gas through a second plasma gas flow path following
plasma arc generation.
In some embodiments, the first swirl pattern is substantially
directed radially inward. In some embodiments, the second swirl
pattern is substantially directed axially into a plasma chamber. In
some embodiments, the plasma arc torch system further includes a
third plasma gas supply source. In some embodiments, the third
plasma gas supply source is distinct from the first and second
plasma gas supply sources.
In some embodiments, each distinct supply source includes a
plurality of gases. In some embodiments, the plasma gas supply
sources include at least one of oxygen, nitrogen, air, and argon.
In some embodiments, the control unit is further configured to
detect an arc transfer following generation of the plasma arc. In
some embodiments, the control unit is further configured to change
at least one of (i) the plasma gas composition, and/or (ii) the
plasma gas flow path. In some embodiments, a third plasma gas flow
path includes portions of both the first plasma gas flow path and
the second plasma gas flow path. In some embodiments, changing the
second plasma gas composition or the second plasma gas flow path is
performed in response to (i) removal of an arc signal from the
plasma torch system, and/or (ii) a time offset from a transfer
sense.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing discussion will be understood more readily from the
following detailed description of the invention when taken in
conjunction with the accompanying drawings. In the following
description, it is understood that a "flow path" can be a path
internal to the torch body. It is also understood that an "arc on"
signal can be equivalent to a "start signal."
FIG. 1 is a schematic diagram of a gas flow system for a plasma arc
cutting system, according to an illustrative embodiment of the
invention.
FIGS. 2A-2D are cross-sectional illustrations of a plasma arc torch
showing several configurations of gas flow paths and consumable
geometries within the plasma arc torch, according to an
illustrative embodiment of the invention.
FIG. 3 shows an exemplary timing diagram for gas pressure and arc
current of a plasma arc cutting system having an improved electrode
life, according to an illustrative embodiment of the invention.
FIG. 4 is an illustration of a computerized control interface for a
plasma arc cutting system having an improved electrode life,
according to an illustrative embodiment of the invention.
FIG. 5 shows a plot of time versus gas pressure and arc current for
a plasma arc cutting system having an improved electrode life,
according to an illustrative embodiment of the invention.
FIG. 6 shows a plot of time versus gas pressure, voltage and arc
current for a plasma arc cutting system having an improved
electrode life, according to an illustrative embodiment of the
invention.
FIG. 7 is a chart showing test results of electrode life for
electrode configurations according to an illustrative embodiment of
the invention compared with a previous configuration.
DETAILED DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a plasma gas flow system 100 for a
plasma arc cutting system, according to an illustrative embodiment
of the invention. The plasma gas flow system 100 includes gas
supply sources 104, 108, 112 and control valves 116, 120 and 124.
Gas can flow in the plasma gas flow system 100 from the gas supply
source 104 to the control valve 116; from the gas supply source 108
to the control valve 120; and/or from the gas supply source 112 to
the control valve 124. Gas can flow to a plasma torch 128 via an
entry point 144 on the plasma torch 128. Gas can flow separately to
the plasma torch 128 via an entry point 148 on the plasma torch
128. Gas can flow into the entry points 144, 148 either
individually or simultaneously. For example, gas can flow from the
control valve 116 through the entry point 144 via a first gas line
132. Alternatively or in addition, gas can flow from the control
valve 124 through the entry point 148 via a second gas line 136.
Alternatively or in addition, gas can flow from the control valve
120 through both of the entry points 144, 148 via a third gas line
140. The third gas line 140 can merge with the first gas line 132
and/or the second gas line 136 such that gas can be delivered to
the torch 128 via entry points 144, 148 simultaneously or near
simultaneously.
In some embodiments, the gas supply sources 104, 108, 112 each
include a plurality of gases. In some embodiments, the gas supply
sources 104, 108, 112 each contain separate containers for
different gases (e.g. nitrogen, oxygen, air, as shown in FIG. 1).
In some embodiments, the gas supply sources 104, 108, 112 include
other gases alternatively or in addition, e.g. argon. Gas supplied
to the plasma torch 128 via the gas lines 132, 136, 140 can include
any combination of gases supplied by the gas supply sources 104,
108, 112. In some embodiments, the plasma gas flow system 100
allows the gas delivered to the plasma torch 128 to be selected
upstream of the control valves 116, 120, 124. In some embodiments,
the plasma gas flow system 100 can select and/or adjust gas
composition delivered to the plasma torch 128 via control of gas
supply sources 104, 108, 112, and/or control valves 116, 120 and
124. Gas composition can be adjusted throughout a cut process based
on the current cut process stage (e.g., pre-flow, arc ignition,
cutting, arc extinction, etc.), with plasma gas flow system 100
manipulating which gas supply sources 104, 108, 112, are fluidly
connected to plasma torch 128 via the selected gas flow
path(s).
In some embodiments, the control valves 116, 120, 124 are
pressure-controlled proportional valves, e.g. mass flow
controllers. In some embodiments, the gas line 132 includes a vent
valve 152. The vent valve 152 can be used to drain gas from the
plasma line 132 when the arc is extinguished. When the arc is
extinguished life can be increased by having low gas flow through
the plasma torch 128. The vent valve 152 can enable gas that would
normally need to flow out the front of the plasma torch 128 to flow
out of the back of the plasma torch 128, thus improving electrode
life. In some embodiments, a vent valve is placed in the plasma
line 136 instead of, or in addition to, the plasma line 132.
FIGS. 2A-2D are cross-sectional illustrations of a plasma arc torch
(e.g. the plasma arc torch 128 as shown above in FIG. 1) showing
several configurations of gas flow paths and consumable geometries
within the plasma arc torch, according to an illustrative
embodiment of the invention. FIG. 2A shows a plasma torch 200A
having an electrode 216, a nozzle 220, and internal flow paths
204A, 208A. Gas can enter the internal flow path 204A via an entry
point 148A (e.g. the entry point 148 as shown above in FIG. 1). Gas
can enter the internal flow path 208A via an entry point 144A (e.g.
the entry point 144 as shown above in FIG. 1). Gas can travel
through the internal flow paths 204A, 208A and exit the plasma
torch 200 at a nozzle orifice 212.
Inside the plasma torch 200A, gas entering through the entry point
148A and traveling through the internal flow path 204A passes
through a set of flow metering holes labeled "1." The set of flow
metering holes 1 can include six holes. In some embodiments, each
of the holes can be spaced equally around a longitudinal axis of
the torch. In some embodiments, each hole in the set of flow
metering holes 1 can have a diameter of approximately 0.018 inches.
The gas then travels along the electrode 216 and passes through a
set of swirl holes labeled "2." The set of swirl holes 2 can be
configured to generate a swirl pattern of the plasma gas. The set
of swirl holes 2 can include twelve holes. In some embodiments,
each of the holes in the set of swirl holes 2 can have a diameter
of approximately 0.0225 inches. In some embodiments, each of the
holes in the set of swirl holes 2 can be angled at about 20 degrees
relative to a radial direction of the plasma torch 200A to generate
a swirling effect. In some embodiments, a combined flow area of the
metering holes 1 is less than the combined flow area of the holes
in set of swirl holes 2. In some embodiments, the spacing between
holes in the sets of holes 1 and/or 2 is equal (e.g. radially) to
encourage uniform flow throughout the torch. In some embodiments,
the sets of holes 1, 2 can each include between six and twelve
holes. In some embodiments, the holes can be angled between about
10 degrees to about 30 degrees relative to a radial direction. In
some embodiments, a greater angle contributes to a greater swirl
strength. In some embodiments, a lower total flow area contributes
to a higher velocity and/or swirl strength for a given gas
flow.
Gas entering through the entry point 144A and traveling through the
internal flow path 208A passes through a set of flow metering holes
in the nozzle 220 labeled "4." The set of flow metering holes 4 in
the nozzle 220 can include six holes. The holes can be equally
spaced around a longitudinal axis of the plasma torch 200A. Each
hole in the set of flow metering holes 4 can have a diameter of
approximately 0.018 inches. In some embodiments, the total flow
area is a significant factor. In some embodiments, the flow area of
the metering holes can be less than the flow area of a downstream
hole pattern. In some embodiments, the spacing between holes should
be equal (e.g. radially) to encourage uniform flow through torch.
The gas then travels within the plasma torch 200A and passes
through a set of holes labeled "3." The set of holes 3 can be
swirl-generating holes. The set of holes 3 can include twelve
holes. Each of the holes in the set of holes 3 can have a diameter
of approximately 0.043 inches. Each of the holes in the set of
holes 3 can be directed radially inward toward the electrode 216.
The set of holes 3 can be provided with an offset to impart a swirl
component to the gas velocity. The set of holes 3 and/or 4 can be a
separate piece permanently assembled to the rest of the nozzle
using an interference fit. An interference fit can be used so that
these holes are added in a separate piece but the user would
receive a single-piece nozzle. Interference can be the minimum
needed to keep the pieces tighter throughout the life of the
nozzle. Either set of holes 3 or 4 can be added to the base nozzle
piece (e.g. the same piece that has the plasma through hole). In
the FIG. 2A embodiment, hole set 3 is in a separate piece and hole
set 4 is in the base piece.
FIGS. 2B-2D show alternate configurations of the internal torch
geometry of the plasma arc torch (e.g. plasma arc torches
200B-200D, or plasma arc torch 100 as shown above in FIG. 1). Each
of these configurations includes two flow paths 204B-D, 208B-D
internal to the torch (e.g. the FIG. 2B configuration includes flow
paths 204B, 208B; the FIG. 2C configuration includes flow paths
204C, 208C, etc.). Gas travels through unique swirl generation
paths having geometries that vary between FIGS. 2B-2D. For FIG. 2B
both swirl flows are generated using geometry located on the swirl
ring component itself. This design also shows the dual plasma flow
concept in use with the vented nozzle technology. For FIG. 2C both
of the flow paths have a significant radial portion of flow through
both swirl holes. For FIG. 2D, the axially directed swirl is
located radially outward from the other swirl gas flow. In FIG. 2D,
the flow arrangement can be opposite to that shown in FIG. 2A, in
which the axially directed swirl is radially inward from the other
swirl flow.
In some embodiments the relative swirl strengths can be different
between swirl generators. An estimate of swirl strength can be
provided by taking an area ratio of the swirl generation holes. For
example, in FIG. 2A, such a ratio could be defined as 12*0.0225
inch diameter to 12*0.043 inch diameter, or about 0.52. An estimate
of relative swirl strength can be calculated by taking a ratio of
the product of the flow area and the swirl offset. For example, in
FIG. 2C this ratio would be the ratio of the product of the number
of holes*the hole area*the offset of the holes. When comparing a
swirl hole pattern that uses an offset to a swirl hole pattern that
uses angled holes, the ratio will be the product of the flow area
and a factor characterizing the tangential velocity component. For
the axially directed swirl holes, this factor can be the sine of
the swirl angle. For a radially directed swirl hole with an offset,
this factor can be approximated by the offset. In some embodiments,
one relatively weak swirl flow path and one relatively strong swirl
flow path are provided. Generally, strong swirls are advantageous
for cutting material. A strong swirl increases arc constriction,
e.g. provides higher energy densities, protects the nozzle from
damage from the arc, and helps stabilize the arc during operation.
This same strong swirl can be detrimental during arc ignition, when
the arc current is substantially below the steady state current. In
this case, the strong swirl could be destabilizing for the arc, and
the rapid arc motion could increase electrode wear. In some
embodiments, the desired ratio of swirl strengths depends on the
geometry of the torch parts and the operating current.
FIG. 3 shows an exemplary timing diagram 300 for gas pressure, gas
composition, and arc current of a plasma arc cutting system having
an improved electrode life, according to an illustrative embodiment
of the invention. The timing diagram 300 shows four sequential time
periods of significance: a preflow period 304 (corresponding to
plasma flow path 2); an arc ignition period 308 (corresponding to
plasma flow path 3); a cutting period 312 (corresponding to plasma
flow path 1); and an arc extinction period 316 (corresponding to
plasma flow path 3). During these time periods the system uses two
input signals to coordinate the gas and current: an arc transfer
signal, provided at time T.sub.transfer; and an arc stop signal,
provided at time T.sub.stop. Other relevant times can be determined
based on a time delay offset relative to the T.sub.transfer and/or
the T.sub.stop signals. For example, the time T.sub.Cutflow
Transition can be a fixed time interval from the time
T.sub.transfer. The times T.sub.EndflowCurrent,
T.sub.EndflowPressure, and/or T.sub.EndflowTime can be a fixed time
interval from the time T.sub.stop.
In the control scheme shown in FIG. 3, a preflow pressure
P.sub.preflow is provided via plasma flow path 2 (corresponding to
the second distinct plasma gas flow pattern, e.g. flowing gas
through a second path internal to the torch body) during the
preflow period 304. In some embodiments, either nitrogen or air is
used in conjunction with plasma flow path 2.
During the arc ignition period 308, the current and pressure can be
increased. Three events can occur at or around time T.sub.transfer:
the pressure can be increased from P.sub.preflow to a first cut
flow pressure P.sub.cutflow1; the current can be increased from
about zero to a nonzero value I.sub.RampUp; and/or the plasma gas
flow path can be switched to plasma flow path 3 (corresponding to a
combination of plasma flow paths 1 and 2, e.g. flowing gas through
first and second paths internal to the torch body). In some
embodiments, either nitrogen, oxygen or air is used in conjunction
with plasma flow path 3. The pressure can increase rapidly and
taper off smoothly toward a constant value. The current can
increase linearly up until a time around or just after time
T.sub.Cutflow Transition.
During the cutting period 312, a workpiece can be cut using the
plasma arc torch. After time T.sub.Cutflow Transition the plasma
flow path can be switched to plasma flow path 1 (corresponding to
the first distinct plasma gas flow pattern), while the pressure can
be increased to P.sub.cutflow2 and the current can attain a steady
value I.sub.steady. In some embodiments, flow path 1 carries oxygen
gas. Generally, when the arc is at full current and is cutting mild
steel, oxygen can be used.
During the arc extinction period 316, the arc can be extinguished
and a cutting operation ceased. A stop signal can be provided at
time T.sub.stop, which can trigger several events relevant to the
ending of a cutting operation. The current can be rapidly (e.g.
instantaneously or near instantaneously) decreased to a current
I.sub.endflow at time T.sub.EndflowCurrent. At or near time
T.sub.endflowPressure, the plasma flow path can be changed to
plasma flow path 3 and/or the pressure can be decreased to
P.sub.Endflow, e.g. in a smoothed manner shown in FIG. 3. At time
T.sub.EndflowTime the current can be decreased (e.g. linearly) to a
current I.sub.Endpoint, after which the current can be
instantaneously or near instantaneously decreased to zero. Also at
time T.sub.EndflowTime, the pressure can smoothly taper off as
shown in FIG. 3. During this time, a vent valve (e.g. the vent
valve 152 as shown above in FIG. 1) can be active. The vent valve
can impact the timing of the pressure decrease during this time. In
some embodiments, when cutting with low plasma currents, the vent
valve can remain closed.
FIG. 4 is an illustration of a computerized control interface 400
for a plasma arc cutting system having an improved electrode life,
according to an illustrative embodiment of the invention. The
interface 400 can include user input fields for key values
described above. For example, FIG. 4 shows a screenshot of an
interface 400 in which a user has set I.sub.steady to 260A and
I.sub.Endflow to 230A using a ramp up step of 10 A/10 ms, a ramp
down step of 9 A/10 ms. The user has set T.sub.EndflowCurrent to 0
MS; T.sub.EndflowCurrent to 0 MS; T.sub.EndflowTime to 500 ms;
T.sub.CutflowTransition to 100 ms; and T.sub.Endflowpressure to 0
ms. The user has set P.sub.preflow to 22 psi; P.sub.Cutflow1 to 80
psi; P.sub.Cutflow2 to 80 psi; and P.sub.Endflow to 70 psi. The
interface 400 shows exemplary magnitudes and time values used
during development; these values are not necessarily optimized and
reflect only a single set of possible values. The interface 400 can
be part of a software platform that allows a user to customize at
least the parameters shown in the interface 400. The interface 400
can be configured to provide instructions to a control unit (not
shown) of the plasma arc cutting system to implement the user-set
parameters.
FIG. 5 shows a plot 500 of time versus gas pressure and arc current
for a plasma arc cutting system having an improved electrode life,
according to an illustrative embodiment of the invention. In FIG.
5, the plot 500 shows that the gas pressure is changed at a time at
which arc transfer is detected; at a time offset from the transfer
sense; and/or at a time at which the "arc on" signal is removed. In
addition, the flow path can be changed at or near each of these
times. The type of gas and/or flow pattern can also be changed at
or near each of these times, as shown in FIG. 5 (e.g. from nitrogen
gas flowing radially--path 2, to nitrogen gas flowing both radially
and axially--path 3, to oxygen gas flowing axially--path 1, and/or
to nitrogen gas flowing both radially and axially--path 3).
FIG. 6 shows a plot 600 of time versus gas pressure, voltage and
arc current for a plasma arc cutting system having an improved
electrode life, according to an illustrative embodiment of the
invention. The change in gas chemistry can be confirmed by
observing the arc voltages. The plot 600 shows the change in
voltages that occurs when the gas chemistry in the region of the
arc changes. This value is important for controlling the process
timing. It is expected that the time of exposure to different gas
types is critical to optimizing electrode life. When tests were
performed with exposure times of less than .about.100 ms, the
electrode life decreased from when the exposure time was around 300
ms, as shown in FIG. 6. When the exposure time approached 1 second,
electrode life decreased.
FIG. 7 depicts a chart 700 showing test results of electrode life
for electrode configurations according to an illustrative
embodiment of the invention compared with a previous configuration.
Tests were conducted based on an existing cutting process used in
the HPR equipment line manufactured by Hypertherm, Inc. The
existing process involved a 260A oxygen plasma, air shield process.
This process uses a single gas flow pattern and swirl generation
device. This process uses air for the preflow gas and changes to
oxygen right at the arc transfer signal. In this process, there is
no gas change or path change at arc extinction. As can be seen from
the chart 700, methods and systems used in accordance with
principles of the current invention can increase electrode life by
about 100% over the standard process. The life for the high current
process 260A is about the same as the 130A process. One of ordinary
skill in the art would have expected that life improvement would be
seen mostly at short cut cycle times. However, the life improvement
was substantial at the medium cut cycle of 20 seconds. The large
improvement in life at medium and long cycles constitutes
unexpected results. Without being bound to a single explanatory
theory, it is likely that the large improvement in life at medium
and long cycles was caused by changes in the steady flow conditions
through the change in gas velocity.
While the invention has been particularly shown and described with
reference to specific preferred embodiments, it should be
understood by those skilled in the art that various changes in from
and detail may be made therein without departing from the spirit
and scope of the invention as defined by the following claims.
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